Q. Sun

First-principles studies on magnetism of Ni clusters coated and alloyed with Pd

Abstract First-principles electronic-structure calculations are performed for the Ni clusters coated and alloyed with Pd. For coated clusters, the magnetic moments of the Ni core are reduced as compared to the bared one. Especially, Ni1Ni12Pd6Pd24Pd12 cluster with Oh symmetry consists of the ferromagnetic Ni core, the ferromagnetic Pd and paramagnetic Pd shells, the induced magnetic moment of Pd in the interface is 0.1μB, qualitatively in agreement with experimental results found in Ni particles coated with Pd. The dependence of magnetic moment on Pd thickness displays an oscillatory behavior, similar to that found in Co clusters coated with Cu or Ag and in Ni/Pd multilayers. For the alloyed clusters, the enhanced magnetism is confirmed, and the mechanism is clarified: which is resulted not only due to the large atomic distance of Pd but also due to electron transfers. The explanations are given for the different magnetic behaviors between coated and alloyed clusters.

First-principles study of ferromagnetic coupling in Zn1−xCrxTe thin film

Using gradient-corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr-doped ZnTe in both bulk and thin-film configurations. Calculations with full geometry optimization for a Zn1−xCrxTe supercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn1−xCrxTe in both bulk and thin film is energetically the most preferable state irrespective of the concentration and∕or site occupation of the Cr atom. The strong hybridization between Cr3d and Te5p states is found to be responsible for the ferromagnetic coupling, in agreement with recent experiments.

First-principles studies on pure and doped C32 clusters

On the basis of first-principles calculations, pure and doped C32 clusters are studied. Among the nine structural isomers, the fullerene structure with D3 symmetry is found to be the most stable. Due to the small size of the C32 cage, Li and Na atoms can be stably encapsulated, while K and Be atoms are not. On encapsulation, the bond length of the H2 molecule is reduced while the vibration frequency is increased. Substitutional doping is more favourable than endohedral doping for Si atoms. Because of the sp2-bonding features of C atoms, the Si atom is also threefold coordinated in substitutional doping; however, the existence of one dangling bond in the Si atom makes this doped heterofullerene reactive at the Si site, and H termination can produce substantial energy gain.

Dimer interactions of magic W@Si12 clusters

Design of stable Si-cage and Si-tube structures is of great importance to the fields of Si nanotechnology and microelectronics. In order to explore the possibility of constructing a Si nanotube by using the metal-stabilized and tube-shaped Si cluster W@Si12 as the basic unit, detailed first-principles calculations are performed to analyse the interactions of the dimers. It is found that the interactions are orientation dependent, and no interactions exist if stacking is along the sixfold axis. However, the W atom can be used as the spacer to link two units, resulting in a metastable structure. Due to the large surface stress in such a small one-dimensional system, sixfold stacking cannot be extended, and thus the trimer is not dynamically stable. Further, the most stable geometry for the dimer is seriously distorted. Consequently, it is difficult to construct a Si nanotube using this W@Si12 tube-shaped unit.

First-principles studies on the adsorptive properties of hydrogenated Si cluster

Abstract The adsorption of Si, O and Al adatoms on the most stable hydrogenated Si cluster (Si 6 H 12 ) has been explored by using first principles. A special attention is paid to the comparisons of the similarity in adsorptions between the hydrogenated Si cluster and H-terminated Si surface: substitutional adsorption configuration is found for Si atom on Si 6 H 12 cluster, similar to that on H-terminated Si surface; Si–O–Si adsorption configuration is found for O atom on Si 6 H 12 cluster, similar to the case on H-terminated Si surface; nonadsorption of Al atom on Si 6 H 12 cluster is found, similar to the very weak adsorption of Al atom on H-terminated Si surface. This similarity in adsorption between hydrogenated Si 6 H 12 cluster and hydrogenated Si surface would be attributed to the fact: in hydrogenated Si 6 H 12 cluster, H atoms have terminated the dangling bonds, and the Si atoms favor the bulk-like sp 3 bonding even in this small cluster.

Nonmetal-metal transition in Ban clusters

Understanding the development of metallic behavior with increasing cluster size has been one of the important questions in cluster science. Based on Kubos criterion and optimized structures with ab initio calculations, the nonmetal to metal transition in Ba clusters is studied. It has been found that the critical size for this transition is around 32, in agreement with the mass spectra experiment.

First-Principles Studies on Pd Intercalated Graphite

Abstract Based on first-principles calculations, structure optimization of Pd intercalated graphite is performed and the equilibrium values for lattice constants are obtained, in agreement with experimental values. In graphite the carbon atoms within the basal plane are bound together by strong covalent σ-bonds, while atoms in adjacent layers are weakly bound by Van der Waals bonds, results in semimetal. Being intercalated with Pd, due to Pd-4d and C-2p hybridizations, partial charge transfers occur from Pd to C, which increase the density of states at Fermi level and make the compound metallic-like; the heat of formation is found to be 2.45eV for per Pd atom intercalation.

Magnetic phase transition and hydrogen solubility in Fe, Co, and Ni

It was observed experimentally that hydrogen affects strongly the magnetic properties of metals and alloys. Recently, we performed a series of first-principles calculations to study the effect of hydrogen on the magnetism of magnetic metals, and we found that the strong hybridization among the orbitals of hydrogen and nickel atoms suppresses the ferromagnetism completely. On the basis of this first-principles calculation, we developed a statistical thermodynamic model and analyzed the experimental results of hydrogen solubility in transition metals Fe, Co, and Ni using the model. We found that our model can describe the temperature dependence of hydrogen solubility in a wide range of solid phase in the stated magnetic metals.

Effects of cluster–cluster interactions on the structure and magnetic properties in (Fe6)2

Abstract The structure and magnetic properties of the coupled Fe 6 :Fe 6 cluster have been systematically studied with the density functional formalism, and the Kohn–Sham equation is solved self-consistently with the discrete variational method (DVM). It is found that as the intercluster distance decreases the clusters are deformed, which results in the sensitive changes of magnetic moments. The cluster–cluster coupling energy indicates that the two clusters are weakly coupled, and the HOMO-LUMO gap of the coupled clusters is so small that the two coupled Fe 6 clusters are nearly metallic, and the metal–nonmetal transition can be realized by coupling and decoupling of clusters.